This invention relates to a method and apparatus for obtaining hot water at a predetermined temperature from a tank such as a hot water heater containing a quantity of water having a variable non-uniform temperature. The invention is particularly suitable for use with a vessel such as a water balloon or flask wherein water is heated by variable external heating energy applied through a heat exchanger in the lower portion of the vessel and by an auxiliary heating device in the vessel upper portion. The hot water may be used directly by occupants of a building as utility water or may be used to heat the building.